Imaging sensor having microlenses of different radii of curvature

ABSTRACT

The present invention provides an image sensor which comprises improved microlenses to cope with different optical requirements for oblique incident light or different components of light. In one embodiment, the image sensor comprises at least two microlenses having different radii of curvature. In another embodiment, the image sensor comprises at least one asymmetrical microlens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image sensor, and more particularly to animage sensor having microlenses of at least two different radii ofcurvature, and to an image sensor having an asymmetrical microlens.

2. Description of the Related Art

Digital imaging devices have been widely used in many electronicproducts nowadays. They are used in, for example, digital cameras,digital video recorders, cellular phones with photographing function,safety-control monitors, etc.

A digital imaging device usually includes an image sensor chip, such asa CCD image sensor chip or a CMOS image sensor chip. For better opticalperformance, an image sensor chip usually includes a layer of multiplemicrolenses, so that incident light may better focus on a focal plane,i.e., within a photodiode layer. The photodiode layer receives photonsand generates electrical signals thereby.

FIG. 1 shows a cross-sectional view of a conventional image sensor. Asshown in FIG. 1, the structure includes a bottom substrate 11, aphotodiode layer 12, an interconnection layer 13 (shown as one metallayer for simplicity, but may include multiple metal layers), apassivation layer 14, a color filter layer 15 which includes multiplered (15R), green (15G) and blue (not shown) segments, a spacer layer 16,and a microlens layer 17 which includes multiple microlenses 171 forfocusing incident light onto the interface between the photodiode layer12 and the substrate 11. Layers above the microlens layer 17, such aslens, package and bond pad layers, etc., are omitted for simplicity.

The conventional method for making such an image sensor withmicrolenses, is to first form a semi-finished substrate with layers11-16 by conventional semiconductor process steps, and then coat aphotoresist layer on the layer 16. The photoresist layer is exposedaccording to a pattern on a photomask, and developed to form multiplesquare segments 172 as shown in FIG. 2. Thereafter, a reflow step istaken, that is, the semi-finished substrate with the photoresist layerthereon is subject to a temperature of above 150 degree centigrade for10 minutes, so that the photoresist layer is partially melted; due toviscosity of the photoresist material, the melted photoresist layer hasthe contour as the microlenses 171 shown in FIG. 1. Next, the substrateis cooled down to form solid microlenses 171.

The above-mentioned conventional image sensor has the followingdrawback. The microlenses 171 are all formed of the same radius ofcurvature. However, light projected onto microlenses at differentlocations, in particular in an image sensor for use in a medium to largesize digital imaging device (mega pixels or above), may have differentincident angles. More specifically, as shown in FIG. 3, light verticallyprojected onto the microlenses at the center area is received by themicrolenses at the peripheral area with a tilt angle, causing a verticalshift of focus. The spot sizes 181 at the peripheral area are notsatisfactory, and the sensitivity of the image sensor is reduced.

U.S. Pat. No. 6,417,022 discloses a method for producing a microlenswith a long focal length, to cope with thick metal layer total thicknessdue to increased number of metal layers. However, in this cited patent,all of the microlenses on a chip are of the same radius of curvature.This cited patent does not describe any solution to the above-mentioneddrawback shown in FIG. 3.

In view of the above, it is desired to provide an image sensor withbetter sensitivity, wherein the radii of curvature of microlenses atdifferent locations are designed in correspondence with differentoptical requirements.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an image sensorcomprising at least two microlenses having two different radii ofcurvature respectively, so as to improve the sensitivity of the imagesensor.

A second object of the present invention is to provide an image sensorcomprising at least one microlens having an asymmetrical lens structure.

A third object of the present invention is to provide methods for makingthe above-mentioned image sensors.

To achieve the foregoing objects, the present invention provides animage sensor comprising at least two microlenses having two differentradii of curvature, wherein a microlens having a smaller radius ofcurvature may be arranged at a central area of the image sensor, and amicrolens having a larger radius of curvature may be arranged at aperipheral area of the image sensor. Or, a microlens having a smallerradius of curvature may be arranged at a location corresponding to afirst color pixel, and a microlens having a larger radius of curvaturemay be arranged at a location corresponding to a second color pixel.

The present invention also provides an image sensor comprising at leastone microlens having an asymmetrical lens structure, which maypreferably be arranged at a peripheral location of the image sensor.

The present invention further provides a method for making an imagesensor, which comprises: providing a semi-finished substrate; coating aphotoresist material on the semi-finished substrate; patterning thephotoresist material into a plurality of subsets, including at least afirst subset and a second subset having different patterns from eachother; and reflowing the photoresist material wherein the first subsetand the second subset form different contours. In this method, thepatterns of the first subset and the second subsets preferably havedifferent clear ratios.

The present invention also provides a method for making an image sensor,which comprises: providing a semi-finished substrate; coating aphotoresist material on the semi-finished substrate; patterning thephotoresist material into a plurality of subsets, in which at least oneof the subsets includes multiple cavities distributed asymmetricallyalong one horizontal dimension; and reflowing the photoresist materialwherein the at least one subset forms an asymmetrical contour.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the invention may be betterunderstood within the context of the Description of the PreferredEmbodiment, as set forth below, with reference to the accompanyingdrawings, wherein:

FIG. 1 shows a cross-sectional view of a conventional image sensor;

FIG. 2 illustrates how the microlenses in the conventional image sensorare made;

FIG. 3 shows the drawback of the conventional image sensor that there isdefocus issue at the peripheral area due to oblique incident light;

FIGS. 4(A) and 4(B) are cross-sectional views showing a first preferredembodiment according to the present invention, and FIG. 4(C) is a topview corresponding to FIG. 4(B);

FIGS. 5(A)-5(D) show a second preferred embodiment according to thepresent invention, wherein FIGS. 5(A) and 5(B) are cross-sectional viewstaken along different cross-section lines of the same image sensor, andFIG. 5(C) is a cross-sectional view taken along the line C-C of FIG.5(D);

FIGS. 6(A) and 6(B) are cross-sectional views showing a third preferredembodiment according to the present invention, and FIG. 6(C) is a topview corresponding to FIG. 6(B); and

FIGS. 7(A)-7(C) show how light focuses better on the focal plane throughthe asymmetrical microlens.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described according to its preferredembodiments and drawings. The drawings are for illustrative purposeonly; the thickness and width in the drawings are not drawn according toscale.

FIGS. 4(A) and (B) are cross-sectional views of a first preferredembodiment according to the present invention, and FIG. 4(C) is a topview corresponding to FIG. 4(B). The left side of the figures shows thestructure of an image sensor at its central area, while the right sideof the figures shows the structure of the image sensor at its peripheralarea. As shown in FIG. 4(A), the microlenses 211 at the central area ofthe image sensor have a smaller radius of curvature than that of themicrolenses 212 at the peripheral area. According to the presentinvention, the radius of curvature of the microlenses at the centralarea is preferably in the range from about 2.00 to about 2.20, while theradius of curvature of the microlenses at the peripheral area ispreferably in the range from about 2.35 to about 2.55.

The structure shown in FIG. 4(A) may be achieved by reducing the volumeof the photoresist material forming microlenses 212 at the peripheralarea as compared with the volume of the photoresist material formingmicrolenses 211 at the central area. As an example, referring to FIGS.4(B) and 4(C), a semi-finished substrate including layers 11-16 is firstprovided. A photoresist material is coated on the semi-finishedsubstrate to form a photoresist layer 21. After the photoresist materialis coated, the central area and the peripheral areas are exposed withdifferent patterns, and developed accordingly. After exposure anddevelopment, the photoresist material at the peripheral area formsmultiple squares or rectangles 202, each of which has several arrays ofcavities 232. As well known by one skilled in this art, this may be doneby properly design the photomask used for the exposure. The cavities 232serve to reduce the volume of the photoresist material in each square orrectangle 202. Although the cavities 232 are shown to have a uniformsquare shape and are aligned one another, it is apparent that they donot necessarily have to be so. It suffices that the cavities 232 help toreduce the volume of the photoresist material, regardless of the shapeand arrangement thereof. However, if it is intended for a microlens 212to be formed by a square or rectangle 202 to have a symmetrical contour,the cavities 232 in this square or rectangle 202 should preferably bearranged symmetrically.

Next, the substrate with the developed squares or rectangles 201 and 202is subject to a temperature above 150 degree centigrade, so that thesquares or rectangles 201 and 202 are melted. Thereafter, the substrateis cooled down, and the microlenses 211 and 212 are formed as shown inFIG. 4(A).

With the structure shown in FIG. 4(A), the image sensor provides abetter optical performance because light incident to the peripheral areaof the image sensor focuses better onto the focal plane.

A second embodiment according to the present invention is shown in FIGS.5(A)-5(D). In addition to providing microlenses of different radii ofcurvature for central and peripheral areas, respectively, it is alsopossible to provide microlenses of different radii of curvature forpixels of different colors, to compensate different wavelengths oflight. FIGS. 5(A) and 5(B) are cross-sectional views taken alongdifferent cross-section lines of the same image sensor, illustrating thecontours of microlenses for red, green and blue pixels, respectively. Asshown in FIGS. 5(A) and 5(B), the microlenses for red pixels have thesmallest radius of curvature; the microlenses for green pixels have thenext smallest radius of curvature; while the microlenses for blue pixelshave the largest radius of curvature. Such an arrangement may be appliedalone, or together with the first embodiment described above; that is,it may be arranged so that the microlenses for red pixels have the samesmallest radius of curvature throughout the image sensor, or, themicrolenses for red pixels at the central area have a smaller radius ofcurvature than that of the microlenses for red pixels at the peripheralarea, and so are the microlenses for the green and blue pixels.Preferably, the microlenses for red pixels have a radius of curvature inthe range from about 2.02 to 2.12 at the central area, and in the rangefrom about 2.37 to 2.47 at the peripheral area; the microlenses forgreen pixels have a radius of curvature in the range from about 2.05 to2.15 at the central area, and in the range from about 2.40 to 2.50 atthe peripheral area; and the microlenses for blue pixels have a radiusof curvature in the range from about 2.08 to 2.18 at the central area,and in the range from about 2.45 to 2.55 at the peripheral area. Themicrolenses for red pixels preferably have a radius of curvature ofabout 0.01 to 0.06 less than that of the microlenses for green pixels,and the microlenses for blue pixels preferably have a radius ofcurvature of about 0.01 to 0.06 more than that of the microlenses forgreen pixels.

The structure shown in FIGS. 5(A) and 5(B) may be formed by a methoddescribed below. Referring to FIGS. 5(C) and 5(D), wherein FIG. 5(C) isa cross-sectional view taken alone the line C-C in FIG. 5(D), asemi-finished substrate including layers 11-16 is first provided. Next,a layer of photoresist material is coated on the semi-finished substrateto form a photoresist layer 31. The photoresist layer 31 are exposed anddeveloped to form multiple squares or rectangles 301, 302 and 303,corresponding to red, green and blue pixels, respectively. By properlydesigning the photomask for exposure, the squares or rectangles 303corresponding to blue pixels have a largest total cavity area; thesquares or rectangles 302 corresponding to green pixels have a lesslarge total cavity area; and the squares or rectangles 301 correspondingto red pixels have no cavity (as shown) or have a smallest total cavityarea (not shown). In the shown embodiment, the squares or rectangles 302and 303 have the same number of cavities 332 and 333, respectively,while the cavities 333 are larger than the cavities 332. However, otherarrangements are also possible, such as that the cavities 332 and 333are of the same size, but the squares or rectangles 303 include morecavities than the squares or rectangles 302. It suffices as long as thesquares or rectangles 303 have a largest total cavity area (clearratio); the squares or rectangles 302 have a less large total cavityarea; and the squares or rectangles 301 have no cavity or have asmallest total cavity area.

Next, the substrate with the developed squares or rectangles 301, 302and 303 is subject to a temperature above 150 degree centigrade, so thatthe squares or rectangles 301, 302 and 303 are melted. Thereafter, thesubstrate is cooled down, and the microlenses 311, 312 and 313 areformed as shown in FIGS. 5(A) and 5(B).

With the structure shown in FIGS. 5(A) and 5(B), whether appliedtogether with the first embodiment or not, the image sensor provides abetter optical performance because different wavelengths of light arecompensated; different components of light incident to the image sensormay focus better.

A third embodiment according to the present invention is shown in FIGS.6(A)-6(C). According to the present invention, it is also possible tocope with oblique incident light by means of asymmetrical microlenses.Such asymmetrical microlenses may be provided at an area where it islikely to receive oblique incident light, such as the peripheral area.Or, it may be arranged so that all the microlenses in an image sensorare asymmetrical, if desired.

As seen from FIG. 6(A), the microlenses 412 have an asymmetrical contour(i.e., asymmetrical along the cross-section line, in which the left sideof each microlens 412 has a smaller radius of curvature than that of itsright side the microlenses 412 may be symmetrical if viewed from adifferent angle). Such an asymmetrical lens structure serve to betterfocus oblique incident light. Referring to FIGS. 7(A) and 7(B), asymmetrical lens does well in focusing vertically incident light, but isnot so well in focusing oblique light. However, as seen from 7(C), lightincident from the left side focuses better onto the focal plane throughthe asymmetrical microlens 412.

A preferred method for forming such asymmetrical microlenses isdescribed below. Referring to FIGS. 6(B) and 6(C), wherein FIG. 6(C) isa top view corresponding to FIG. 6(B), a semi-finished substrateincluding layers 11-16 is first provided. Next, a layer of photoresistmaterial is coated on the semi-finished substrate to form a photoresistlayer 41. The photoresist layer 41 are exposed and developed to formmultiple squares or rectangles 402. As shown in FIG. 6(C), in thisembodiment, each square or rectangle 402 includes several arrays ofcavities 432, in which the cavities 432 at the right side of each squareor rectangle 402 are denser. However, other arrangements are alsopossible, such as providing larger cavities at the right side of thesquare or rectangle 402. It suffices as long as the squares orrectangles 402 have an uneven distribution of the photoresist material,in which the portion with less photoresist material will form a lenspart with a higher radius of curvature.

The substrate with the developed squares or rectangles 402 is subject toa temperature above 150 degree centigrade, so that the squares orrectangles 402 are melted. Thereafter, the substrate is cooled down, andthe asymmetrical microlenses 412 are formed as shown in FIGS. 6(A) and7(C).

The third embodiment may be applied alone, or together with either orboth of the first embodiment and the second embodiment. As describedabove, the asymmetrical microlens helps to better focus oblique light.

The preferred embodiments of the invention have been described above toillustrate the spirit of the invention rather than to limit the scope ofthe invention. Substitutions and modifications may be made to steps,materials, structures and other aspects of the invention, as apparent tothose skilled in the art. As an example, after exposure and development,the photoresist material needs not be of a square or rectangle shape,but may be of any shape. As another example, the layers under themicrolens layer may be arranged otherwise. Therefore, all suchsubstitutions and modifications are intended to be embraced within thescope of the invention as defined in the appended claims.

1. An image sensor comprising at least two microlenses having differentradii of curvature.
 2. The image sensor as claimed in claim 1, whereinsaid at least two microlenses includes a first microlens having asmaller radius of curvature located at a central area of said imagesensor, and a second microlens having a larger radius of curvaturelocated at a peripheral area of said image sensor.
 3. The image sensoras claimed in claim 2, wherein said first microlens has a radius ofcurvature in a range from about 2.00 to about 2.20.
 4. The image sensoras claimed in claim 2, wherein said second microlens has a radius ofcurvature in a range from about 2.35 to about 2.55.
 5. The image sensoras claimed in claim 1, wherein said at least two microlenses includes afirst microlens having a smaller radius of curvature corresponding to afirst color, and a second microlens having a larger radius of curvaturecorresponding to a second color.
 6. The image sensor as claimed in claim1, wherein said at least two microlenses includes a first microlenshaving a smaller radius of curvature corresponding to a red pixel, asecond microlens having a less smaller radius of curvature correspondingto a green pixel, and a third microlens having a larger radius ofcurvature corresponding to a blue pixel.
 7. The image sensor as claimedin claim 6, wherein said first microlens has a radius of curvature ofabout 0.01 to 0.06 less than that of said second microlens.
 8. The imagesensor as claimed in claim 6, wherein said third microlens has a radiusof curvature of about 0.01 to 0.06 more than that of said secondmicrolens.
 9. The image sensor as claimed in claim 6, wherein said firstmicrolens has a radius of curvature in a range from about 2.02 to 2.12;said second microlens has a radius of curvature in a range from about2.05 to 2.15; and said third microlens has a radius of curvature in arange from about 2.08 to 2.18.
 10. The image sensor as claimed in claim6, wherein said first microlens has a radius of curvature in a rangefrom about 2.37 to 2.47; said second microlens has a radius of curvaturein a range from about 2.40 to 2.50; and said third microlens has aradius of curvature in a range from about 2.45 to 2.55.
 11. The imagesensor as claimed in claim 1, wherein said at least two microlensesincludes a first microlens having a symmetrical lens structure, which islocated at a central area of said image sensor, and a second microlenshaving an asymmetrical lens structure at least along one horizontaldimension, which is located at a peripheral area of said image sensor.12. An image sensor comprising at least a microlens having anasymmetrical lens structure at least along one horizontal dimension. 13.A method for making an image sensor, comprising: providing asemi-finished substrate; coating a photoresist material on saidsemi-finished substrate; patterning said photoresist material into aplurality of subsets, including at least a first subset and a secondsubset having different patterns from each other; and reflowing saidphotoresist material wherein said first subset and said second subsetform different contours.
 14. The method as claimed in claim 13, whereinat least one of said first and second subsets includes multiplecavities.
 15. The method as claimed in claim 13, wherein said patternsof said first and second subsets have different clear ratios.
 16. Themethod as claimed in claim 13, wherein said first subset forms a firstcontour having a smaller radius of curvature and is located at a centralarea of said image sensor, and said second subset forms a second contourhaving a larger radius of curvature and is located at a peripheral areaof said image sensor.
 17. The method as claimed in claim 13, whereinsaid first subset forms a first contour having a smaller radius ofcurvature corresponding to a first color, and said second subset forms asecond contour having a larger radius corresponding to a second color.18. The method as claimed in claim 13, wherein said subsets includessaid first, said second, and a third subsets, said first subset forminga first contour having a smaller radius of curvature corresponding to ared pixel, said second subset forming a second contour having a lesssmaller radius corresponding to a green pixel, and said third subsetforming a third contour having a larger radius corresponding to a bluepixel.
 19. The method as claimed in claim 13, wherein said first subsetforms a symmetrical contour and is located at a central area of saidimage sensor, and said second subset forms an asymmetrical contour atleast along one horizontal dimension and is located at a peripheral areaof said image sensor.
 20. A method for making an image sensor,comprising: providing a semi-finished substrate; coating a photoresistmaterial on said semi-finished substrate; patterning said photoresistmaterial into a plurality of subsets, in which at least one of thesubsets includes multiple cavities distributed asymmetrically along onehorizontal dimension; and reflowing said photoresist material whereinsaid at least one subset forms an asymmetrical contour.